1. Field of the Invention
The present invention relates to a method of producing an optical fiber preform capable of improving impact-resistance and heat-shock resistance. The present invention also relates to an optical fiber preform produced by the same method. The present invention further relates to a glass rod for producing an optical fiber, which is appropriately used in the same method.
Priority is claimed on Japanese Patent Application No. 2008-201299, filed Aug. 4, 2008, the content of which is incorporated herein by reference.
2. Description of Related Art
In the production of an optical fiber preform, a glass rod is generally prepared to have a structure corresponding to a core of an optical fiber or a core and a clad deposited on the core of an optical fiber (such a glass rod is hereinafter, referred to as a glass rod). Next, a porous glass preform is obtained by growing and depositing porous silica glass particles (soot) on the periphery of the glass rod, where the porous silica glass particles are formed by flame hydrolysis, thermolysis or the like. Next, the porous glass preform is heat treated in a heating furnace to perform sintering, and where necessary, dehydration, thereby vitrifying the silica glass porous body to a transparent glass and obtaining an optical fiber preform.
For example, VAD (Vapor Axial Deposition) Method, OVD (Outside Vapor Deposition) Method or the like may be used as a method of producing a silica glass porous body. In the VAD method, a porous glass preform is obtained by depositing soot on a tip of a seed rod, and growing the soot deposit in the vertical direction. In the OVD method, a porous body is obtained by depositing a soot on a periphery of a glass core. The glass core used in the OVD method may be a glass rod formed by vitrifying a silica glass porous body formed by the VAD method or the like, or a glass rod formed by drawing the vitrified silica glass porous body.
For example, the silica glass porous body may be vitrified by suspending the porous glass preform along the vertical direction, and moving the porous silica glass preform relative to the heater in the heating furnace or moving the heater relative to the porous silica glass preform in the heating furnace, thereby vitrifying the silica glass porous body from one end to the other end.
In the above-described vitrification method, temperature in the heating furnace is controlled by controlling an input power of the heater based on a surface temperature of the heater measured by a radiation thermometer of the like, and on a preliminary identified relationship between the input power and the heating temperature, or other properties.
As explained above, in the vitrification method, the temperature of vitrifying the porous glass to a transparent glass is controlled to be in an appropriate range by controlling a temperature of the heating furnace. In general, before vitrifying the porous glass by sintering, dehydration process is performed so as to remove moisture included in the soot. A temperature used in the dehydration process is about 1000° C. On the other hand, vitrification by sintering is performed at relatively higher temperature of about 1500° C.
In the production of an optical fiber preform, a preform is produced to have a valid portion and invalid portions positioned on both ends of the valid portion. The valid portion denotes a portion usually worked to an optical fiber. In a porous glass preform, two end portions constitute the invalid portions, and a main portion lying between the two end portions constitutes the valid portion.
In the above-described production of an optical fiber preform, there is a case where the glass rod is constituted to have a first rod and dummy rods fusion-bonded to both ends of the first rod, and the silica glass porous body is deposited on a periphery of the glass rod. In this case, along an axial direction (direction of the center axis), a portion of the dummy rod, that is, a portion from the fusion-bonded boundary to a tip end, constitutes an invalid portion, and a portion of the first glass rod constitutes the valid portion. In this case, after the production of the optical fiber preform, a dummy rod may be removed from a partial portion of the invalid portion, and reused in a production of an optical fiber preform.
Mass production of optical fiber preforms and optical fibers are proceeding in accordance with recently increasing demand. Therefore, there are requirements for increasing the size of a preform, shortening of a time of the production of the preform, and enhancement of drawing speed, or the like. In particular, it is effective to increase the size of the preform to decrease a production cost of an optical fiber per unit length. Therefore, various researches have been carried out in order to enlarge the sizes of optical fiber preforms.
For example, Patent Reference 1 (Japanese Unexamined Patent Application, First Publication No. 2003-81657) discloses a method of optimizing a vitrification process in relation with a method of vitrifying a large sized porous glass preform. Patent Reference 1 proposes a method of optimizing a vitrification temperature, moving speed of the preform, and gas supply conditions with the intention of suppressing the occurrence of a non-vitrified unsintered portion, and elongation of the preform caused by an excessive vitrifying temperature.
In the method described in Patent Reference 1, while moving the porous glass preform in a heating region, the heating temperature of the porous glass preform and the moving speed of the preform in a heating region are decreased in a tail end of the preform compared with a top end of the preform heading to its moving direction. In this case, in the tail end opposite to the top end, radial thermal distribution is generated in a dummy rod (glass rod in an invalid portion) supporting the silica glass porous body such that the temperature of the circumferential portion is higher than the temperature of the central portion. In a production of a large-sized porous glass preform, temperature difference along the radial direction is further expanded since the dummy rod is required to have a large diameter so as to support a silica glass porous body of large mass. Where a large temperature difference along a radial direction is generated in the dummy rod, viscosity difference is generated depending on the temperature difference, thereby generating large difference of the residual strain in the glass along the radial direction. As a result, shock resistance of the dummy rod is deteriorated. Therefore, there was a problem that the dummy rod tended to deform by cracking, failing (breaking), or the like by the application of vibration or impact during removing the produced optical fiber preform from the production apparatus and carrying the optical fiber preform. In addition, in the case of reusing such a dummy rod in production of an optical fiber preform, because of the low shock resistance of the dummy rod, there was a problem that a rapid heating of the dummy rod tended to occur deformation such as cracking or breaking. Such a problem is prominent when a large tensile stress remains on the periphery of the dummy rod depending on the radial thermal distribution of the dummy rod. Reprocessing of the dummy rod, for example by annealing, to release the residual strain before reusing the dummy rod may be taken into consideration as a method for inhibiting the deformation of the dummy rod at the time of reusing the dummy rod However, in such a case, there is a possibility of deforming the dummy rod during the reprocessing, and an additional process increases the production cost. In addition, there was a problem that the deformation of the dummy rod was further magnified by the influence of increased mass of the silica glass porous body.
Based on the above-described circumstance, an object of he present invention is to provide a method of producing an optical fiber preform that includes vitrifying a silica glass porous body and enhances shock resistance and heat-shock resistance of the glass rod in the invalid portion, and can be applied to a large sized optical fiber preform. Another object of the present invention is to provide an optical fiber preform produced by the same method, and a glass rod which is appropriately used in the production of the optical fiber preform.